Tea is one of the world's favorite beverages and it is used by many people who want to lose weight or maintain their health. Both black and green tea contain caffeine and the soothing amino acid L-theanine. Black tea contains theaflavins, thearubigins and catechins, however they are oxidized so some of the benefits may not be as potent as those of unoxidized green tea. However, a study showed that both black and green tea significantly changed the bacteria in the gut, altering energy metabolism.